New Prime Minister Bill English has revealed he would vote in favour of same-sex marriage if another vote was held, saying he has witnessed the positive impact it has had on gay couples in New Zealand.

English, in his first press conference after being selected as National Party leader, also said he would not use his Prime Ministerial position to push through or lobby for socially conservative changes in New Zealand.

He was chosen as the party's new leader after a short caucus meeting this morning, and was sworn in as Prime Minister at Government House this afternoon.

Speaking to reporters after the caucus meeting, English told reporters he was an "active Catholic and proud of it". But while his Catholic faith was an important influence, it did not define him, he said.

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He said his opposition to euthanasia and abortion remained the same. But his position on gay marriage had changed.

"I'd probably vote differently now on the gay marriage issue. I don't think that gay marriage is a threat to anyone else's marriage."

English voted against the legalisation of same-sex marriage in New Zealand in 2012. It passed into comfortably with the support of two-thirds of MPs.

Asked what had changed his view, English said: "Just seeing the impact it has had for couples and the fact that it doesn't erode marriage.

"In some ways, it's an affirmation of the concept.

"But again, I don't intend to use the position to try and influence those issues. They've been dealt with by the Parliament and by the processes of law and I accept that."

English's U-Turn was welcomed by the Human Rights Commissioner Richard Tankersley, who said the courage to challenge your own views publicly was "a rare but valuable quality in any leader".

The new Prime Minister's comments were also criticised by some MPs, who said his support for the law change was too little, too late.

Labour's finance spokesman Grant Robertson, who is openly gay, tweeted that English had the opportunity to support same-sex marriage in 2012 but had not taken it. He said actions spoke louder than words.

English also said that if a law change relating to euthanasia came before Parliament, National MPs would have a conscience vote.

Parliament's Health Committee is considering whether assisted dying should be legalised in New Zealand. Outgoing Prime Minister John Key has already said the National-led Government would not change the law regardless of the committee's findings.

Act Party leader David Seymour also has a bill in the ballot which would legalise voluntary euthanasia in some circumstances.